The present invention relates to a deposition method for producing silicon carbide (SiC) high temperature semiconductors and more particularly to a method for producing SiC high temperature semiconductor materials using a new fluidized bed deposition system. The articles of epitaxial SiC are formed by deposition on single crystal SiC substrates. The chemical vapor deposition (CVD) in a fluidized bed is practiced with SiC or Si particles, hydrogen gas in a mildly bubbling mode and reactants, such as methyltrichlorosilane or a mixture of SiH.sub.4 /C.sub.3 H.sub.8.
Most semiconductors have been made of pure silicon. These silicon chips, however, are adversely affected by temperatures above 300.degree. C. Semiconductors made of SiC with a large band gap e.g., 2.3 eV for Beta SiC and 2.9 eV for 6H SiC, should be capable of enduring temperatures around 900.degree. C. Such semiconductors could provide the capability to place electronic packages inside turbine engines to monitor and control the engine to a degree never before possible.
Silicon carbide devices would also improve the instrumentation for military aviation, nuclear-powered generators, and other high-temperature environments. Moreover, SiC also appears to be promising as a very high frequency semiconductor. Its refractory nature, inertness and high energy gap allow high power dissipation and superior reliability.
Early silicon carbide efforts were a failure because sufficiently pure crystals of silicon carbide were difficult to make and experimental production methods were not adequate. Presently, there is no one practical process whereby single SiC crystals of sufficient size, purity and perfection could be repeatably grown (see Nieberding, "Researchers Develop Long-Sought SiC Crystal Growth Technique," Industrial Research & Development, P. 148, September 1983). One of the major fundamental problems is the uniformity of deposition. It is difficult to uniformly heat the substrate via inductive coupling through the supporting graphite susceptor in the conventional reactors (see Ban, "Novel Reactor for High Volume Low-Cost Silicon Epitazy," JI. of Crystal Growth, 45, P. 97, 1978). A slight temperature gradient lead to many forms such as polytypes of SiC crystal. Also, the difficulty in maintaining a well-balanced environment and the conditions imposed by a mass-transfer limiting mechanism make it hard to yield a uniform coating.
The most well-known method of making SiC crystals is the chemical vapor deposition (CVD) of volatile silicon and carbon compounds in the presence of hydrogen (See Inomata et al, "Growth of SiC Single Crystals from Silicon Vapor and Carbon," Silicon Carbide--1973, P. 133, Proceedings of the 3rd International Conference on SiC, Florida, September 1973, ed. by R. C. Marshall et al., Univ. of So. Carolina Press, and, Wessells et al, "Epitaxial Growth of Silicon Carbide by Chemical Vapor Deposition," Silicon Carbide--1973, P. 25, 1973). .beta.-Silicon carbide crystals can be prepared via pyrolysis of gaseous compounds at a heated carbon or silicon substrate around 1500.degree. C. by the so-called van Arkel process (See Knippenberg, "Growth Phenomena in Silicon Carbide," Philips Res. Report 18, No. 3, 161-274, 1963). The product can be either polycrystalline or single crystal, depending upon the size and quality of the substrate. In this type of CVD reactor, as cooler gas flows over the substrate, its leading edge will be at a slightly lower temperatures than the exit edge. This will result in an uneven deposition thickness of different polytypes due to the sensitivity of SiC crystalline structure towards temperature. The nonequivalency of water positions with respect to the gas flow makes it difficult to attain thickness uniformity due to the reactant depletion at downstream positions. The power efficiency is low due to large radiative heat losses from the exposed susceptor surfaces, and the high temperature substrate incurs a large radiation energy loss to the empty surrounding. The throughput for this process is low due to the limitation of reactant concentration in order to prevent the formation of homogeneous fines, which cannot be handled in the conventional CVD type of deposition. The chemical efficiency is low, with some 60-70% of the incoming reactant gas being exhausted unutilized. Moreover, it is a batch process, involving high labor costs.
Other conventional CVD methods are disclosed in U.S. Pat. No. 3,520,740 to Adammiano, U.S. Pat. No. 3,925,577 to Fatzer et al and British Pat. No. 955,700, namely a free-spaced vessel-type process. However, it is difficult to expect to obtain the high temperature semiconductors as that in a fluidized bed of the present invention.
Another well-known method for the production of SiC crystals utilizes sublimation. First, commercial grade silicon carbide is prepared by reacting coal and sand in an electric furnace (See Munch, "Silicon Carbide Technology for Blue-Emitting Diodes," JI. of Electronic Materials, 6, No. 4, P. 449, 1977). Then, the impure hexagonal silicon carbide is purified through a small-scale sublimation technique, invented by Lely, at about 2600.degree. C. Not only is this method of energy intensive and capable only of small scale production because of heat transfer limitations but also the product is frequently contaminated by the impurities in the starting raw SiC material and in the graphite insulation. Because precise control of a process at temperatures as high as 2600.degree. C. is very difficult, SiC crystals grown by sublimation are made up of a mixture of many polytypes. Consequently, the final product usually consists of small, irregularly shaped crystals with unpredictable semiconductor properties.
Another further well-known method for producing SiC crystals utilizes crystallization from solution, e.g. the liquid phase epitaxial. The biggest difficulty here is to contain a silicon/carbon melt at a temperature of several hundred degrees higher than the silicon melting point, i.e. 1800.degree. C. v. 1420.degree. C., without producing any contamination. The materials' problem in a sizable high-temperature liquid container and the associated wall contamination presents intractable difficulties.
Combinations of the above methods have also been used to improve the purity and the quality of resulting crystals. Nevertheless, they all share the inherent drawbacks of extensive energy consumption, and small-scale batch operation. These shortcomings are considered to severely hinder the realization of SiC high-temperature semiconductor technologies on a large-scale.
Accordingly, it is an object of the present invention to overcome the above-noted disadvantages and to provide a fluidized bed deposition system for producing SiC high-temperature semiconductor materials.
Another object of the present invention is to provide a SiC system to yield improved uniform deposition over the conventional CVD reactor.
It is a further object of the present invention to provide a low-cost process for producing large-volume semiconductor-grade SiC crystals.
It is still another object of the present invention to provide time-phased studies on characterizing the structural, chemical, and electrical performances of the deposition of high-temperature semiconductor materials.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.